1. Field of the Invention
The present invention relates to a truing apparatus for truing (shape correcting) a polishing pad used in a wafer polishing apparatus.
2. Description of the Related Art
In general, in a wafer polishing apparatus for polishing the surface of a semiconductor wafer, polishing is performed in such a way that a polishing pad is attached onto the top surface of a disk-shaped platen (fixed plate), one or more wafers are placed on this platen, and a fine diamond slurry is supplied between the polishing pad and the wafers while these wafers are being forcedly rotated on the polishing pad by a carrier. The above-described polishing pad may consist of a velour type pad having a polyurethane resin impregnated into a polyester nonwoven cloth; a suede type pad having a foaming polyurethane layer formed on a polyester nonwoven cloth by using this cloth as a base material; or a polyurethane sheet which is foamed in a closed-cell manner.
It is preferable that the surface of this type of polishing pad be as flat as possible. However, in practice, it cannot be avoided that some irregularities will occur on the surface of the pad due to unevenness of the pad thickness arising from a pad manufacturing step or due to the thickness of a bonding layer required to attach the pad to a platen. Therefore, problems arise, for example, the flatness of the polishing pad exerts an influence upon the flatness of the wafer 1.
Also, there is a drawback in that the surface of the polishing pad becomes toughened, for example, becoming napped or wavy, while wafer polishing is repeated, and thus wafer polishing accuracy is decreased due to the irregularities caused by the roughening.
Therefore, to make the polishing pad flat, a method is disclosed in, for example, Japanese Patent Laid-Open No. 64-71661, in which a correction ring having a diamond pellet stuck on the end surface thereof or having diamond abrasive grains electrodeposited on the end surface thereof is placed on the polishing pad, and the polishing pad and the correction ring are moved relative to each other to polish the surface of the polishing pad, thus increasing the flatness of the surface of the polishing pad. In this case, the desirable grain size of the diamond abrasive grains is assumed to be #400 to #3000.
Also, disclosed in Japanese Patent Laid-Open No. 4-343658 is an apparatus having a wafer mounted on the bottom surface of a top ring and having this wafer pressed against a polishing cloth attached onto the top surface of a turntable and polished, wherein a correction ring is mounted coaxially on the outer circumference on the top ring, the wafer is polished and at the same time the polishing cloth is ground by the correction ring in order to remove the surface roughness.
Disclosed in Japanese Patent Laid-Open No. 4-364730 is an arrangement in which the roughness of a polishing cloth is corrected by making a nylon brush, a pallet having diamonds electrodeposited thereon, or a cutter rotate on the polishing cloth attached to the fixed plate of the wafer polishing apparatus while the nylon brush, the pallet or the cutter is moved forward or backward.
Since the method of truing a polishing pad described in each of the above publications is concerned with grinding the surface of the polishing pad, the thickness of the polishing pad is naturally reduced and may become insufficient after truing is performed a number of times, in which case the polishing pad must be replaced. To replace a polishing pad, a wafer polishing operation must be interrupted, impeding production efficiency. Thus, it is a matter of course that the longer the service life of the polishing pad, the better.
The inventors of the present invention conducted an experiment from the viewpoint of obtaining a sufficient truing effect while keeping the polishing amount of the polishing pad to a minimum. They found that the two means described below make it possible to obtain a high truing effect and to limit the reduction in the amount of the thickness of the polishing pad.
1. A truing grinding wheel is made tiltably movable with respect to its rotational axis so that even when a slight slope is present on the surface of the polishing pad, the polishing surface of the truing grinding wheel is brought into parallel abutment with the polishing pad in such a manner as to follow the slight slope. The inventors of the present invention found that in the conventional construction in which the truing grinding wheel is rigidly fixed with respect to its rotational axis, excessive localized pressure occurs on the abutment surface between the polishing pad and the truing grinding wheel in the truing step, and the polishing pad is ground more than necessary.
2. As abrasive grains which form an abrasive layer of a truing grinding wheel, ultra-abrasive grains of #60 to #230, which are too coarse from the common experience of the person skilled in the art, is used. In the past, it has been thought that use of such ultra-coarse abrasive grains causes the surface of the polishing pad to be roughened. On the contrary, however, the experiment by the inventors of the present invention revealed that this type of coarse abrasive grains does not grind the material of the polishing pad at a predetermined abutment pressure and shows a flattening effect.